Inline data is compressed unconditinally even when results in increased size

[Hanabishi <i.r.e.c.c.a.k.u.n+kernel.org@gmail.com>]

Hello.

I noticed that a btrfs volume with compression enabled (e.g. 'compress=zstd') seems to compress inline data unconditionally, even in situations when compression is ineffective and only leads to increase in size.

It is easily reproducible on small files:

# echo 'a file with test content' > test.txt
# sync
# compsize test.txt
Processed 1 file, 0 regular extents (0 refs), 1 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL      172%       43B          25B          25B
zstd       172%       43B          25B          25B

Same file when no compression enabled:

Processed 1 file, 0 regular extents (0 refs), 1 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL      100%       25B          25B          25B
none       100%       25B          25B          25B

It's not a big deal on practice, but shouldn't it detect that the file is uncompressible and fall back to no compression?
Also, if I remember correctly, it wasn't the case a couple of kernel releases back.